It is found that a major source of the noise generated during takeoff and landing of an aircraft comes from its retractable undercarriages. Specifically, a retractable undercarriage comprises a set of wheels carried by a structure that is designed to be movable so as to enable the entire undercarriage to be retracted once the aircraft is in flight.
Given the various constraints that such an undercarriage needs to satisfy, such as withstanding forces while landing and taxiing, or the necessity of ensuring that the undercarriage can be deployed under all circumstances, the structure carrying the wheels is relatively complex.
Such a structure comprises a central leg that is pivotally mounted about a top transverse axis, and that carries the set of wheels at its bottom end. The structure also includes a plurality of bars or rods that are hinged to the leg, to the structure of the aircraft, and to one another, by as many hinges, each of which includes a pin about which the elements that are hinged together pivot. In order to reduce weight, the pins are generally hollow.
The undercarriage thus includes two rods forming a side brace for locking the leg in its deployed position, and, when the undercarriage is a nose undercarriage, two other rods that constitute a scissors linkage capable of transmitting steering movement about a vertical axis from an upper portion to the set of wheels in order to steer the aircraft.
Furthermore, the undercarriage also includes a series of flexible hydraulic and other pipes that are provided in redundant manner, going from a top portion of the undercarriage down to brakes that are fitted to the set of wheels, the pipes serving to convey the hydraulic power needed for braking.
The flexible pipes are secured to the components of the structure at particular points, e.g. by means of collars, in such a manner as to ensure they retain sufficient flexibility to enable the undercarriage to be deployed and to be retracted without damaging the pipes.
Specifically, such a retractable structure with its pieces of equipment presents a geometrical shape that is complex with a multitude of openings, baffles, and passages that generate a very large amount of aerodynamic noise when a stream of air traveling at high speed passes therethrough.
Furthermore, reducing the noise generated by an aircraft while landing and taking off nowadays constitutes a major concern.